This invention relates to lubricants for use in the processing of plastics, and in particular, to lubricants for use during the shaping of thermoplastic polymers.
In the known methods for processing thermoplastic polymers, shaping is carried out under high temperature and pressure loads. The heated plastic mass tends to adhere to heated machinery parts, especially when the processing of the thermoplastic materials involves injection molding, extruding, calendering or rolling. The adhering material thus has a longer dwell time in the machines, as a result of which thermal degradation of the thermoplastic polymer occurs, which may thus require interruption of a processing operation. When polyvinyl chloride is processed, a result of the thermal degradation in combination with the limited useful life of typical stabilizers, may be the splitting off of hydrochloric acid.
In order to avoid the foregoing processing difficulties, auxiliary processing agents are typically added to the plastic powder in addition to the customary stabilizers, the agents tending to facilitate the flow of the melted mass and to eliminate adhering of the plastic to the heated surfaces of the machinery parts.
Typical auxiliary processing agents include lubricants and flowing aids which are added to the thermoplastic polymer. In the case of lubricants, one may differentiate between internal and external lubricants. The internal lubricants are intended to serve an increase in melting velocity and in their case a reduction in internal friction, thus a certain internal lubrication is important, which presupposes an adequate compatibility of a lubricant with a plastic. External lubricants, sometimes also referred to as parting agents, serve to prevent adhering to the hot machinery parts and in such a case, less compatibility with a plastic is required, so that the lubricant will difuse between the plastic material and the heated machinery parts, and thus cause a lubricating effect at the interphase between the metal and the melt.
Wax esters, such as the cetyl ester of palmitic acid, fatty alcohols, as well as fatty acid partial esters of glycerine, such as glycerine mono-oleate, are known to be excellent internal lubricants. Among the external lubricants with good parting effect are fatty acids, fatty acid amides, fatty acid esters, lower alcohols, natural parafin hydrocarbons, and hardened glycerides. Polyalkylene oxides of low 1,2-alkylene oxide have already been proposed as lubricants. In German Pat. No. 1,133,544, the use of polyethylene oxides as lubricants in the manufacture, without plasticizer, of rolled film made of vinyl chloride polymers and copolymers is described. In addition to polyethylene oxide, the corresponding British Pat. No. 887,353 also mentions polypropylene oxide, as well as copolymers of ethylene oxide and propylene oxide as suitable lubricants. Japanese Pat. No. 43-26088 teaches the use of polyisobutylene oxide, with a reduced viscosity of at most 1.5, as a processing aid in the processing of polyvinyl chloride. However, lubricants such as the foregoing are suitable only for narrow fields of application, so that the previously utilized conventional lubricants, such as wax esters, fatty alcohols, fatty acid amides, fatty acid esters, as well as their combinations, continue to find wide application.
Selection and dosage of the lubricant depend primarily upon the temperature and pressure stresses prevailing during processing, but also upon the characteristics of the other components of the mixture, as well as upon the desired characteristics of the finished product. For practicing certain processing methods, one may be forced to use combinations of several lubricant types, as each lubricant is typically designed for accomplishing only one specific goal. However, despite the possibility of improving the characteristics of one lubricant through combination thereof with another lubricant, it is unavoidable that products with overlapping characteristics will result from the mixture. The necessity of developing a special mixture for a special case, which is by no means a rare occurrence, involves considerable expenditure for the processor. Nevertheless, even if lubricant combinations are utilized, incompatibility effects, such as "blooming," cannot be avoided, if one maintains the minimum quantities of lubricant required to attain adequate freedom from adhesion.
An object of the present invention, therefore, is to provide a lubricant which, in its actions as an internal and external lubricant, can be adjusted in such a way that there will be no risk of incompatibility, and optimal processing conditions will be made possible.